Peptide fusion inhibitors (PFIs) are anti-retroviral compounds that prevent a conformational change in the HIV-1 envelope glycoprotein (Env) that are necessary for membrane fusion and entry. These conformational changes involve molecular interactions between two helical domains of gp41 (heptad repeat HR1 and HR2) that come together to form a six-helix bundle. PFIs are proposed to act as a competitive inhibitor that interact with the heptad repeats when they are in an exposed prehairpin state and prevent folding into the fusion active six-helix bundle conformation. Our hypothesis is that the rate of transition into the six-helix bundle is governed by the stability of heptad repeats as well as by inter-helical packing interactions. A corollary to this hypothesis is that an accelerated rate of transistion from prehairpin intermediate to six-helix bundle formatin would reduce the window of opportunity for a PFIs to interact with its target on Env. To test these hypotheses, I will: i) determine whether amino acid residues in HR2 contribute to a PFI-insensitive phenotype using chimeric Env pseudotypes, ii) determine whether intermolecular interactions contribute to PFI insensitivity by analyzing the ability of PFI analogues containing strain-specific sequences to inhibit virus entry, iii) determine whether PFI sensitivity is reflected in the rate of virus entry, and iv) determine the extent to which the window of susceptibility to PFI is shortened for patient-derived viruses. Completion of these studies will provide critical information about the potential for development of viral resistance to PFI as well as detailing the molecular interactions that drive fusion and entry.